As one display technique for a three-dimensional image, there is a frame sequential technique. The frame sequential technique is a technique of alternately displaying a left-eye image and a right-eye image, allowing a stereoscopic image to be visually recognized (see Patent Literature 1 (JP2012-244426A)).
In the frame sequential technique, a set of continuous left-eye images and right-eye images is recognized as a stereoscopic image, and therefore, for example, in the case where an image signal has 1080/60p, the left-eye image and the right-eye image are alternately displayed for each frame at 120 Hz, which is twice 60 Hz (two-time speed).
When the image is displayed at a two-time frame rate in the case of image signals of 1080/30p and 1080/24p, the left-eye image and the right-eye image are alternately displayed at 60 Hz and 48 Hz, respectively. In this case, a long frame drawing time causes a phenomenon called a cross talk in which, at the time of displaying an image for one eye, an image for the other eye appears remains displayed. Therefore, in the case of the image signals of 1080/30p and 1080/24p, the display is performed at a four-time speed and a five-time speed, respectively, and the left-eye image and the right-eye image are alternately displayed at 120 Hz. Thereby, the occurrence of the cross talk is prevented.
By the way, some image display devices include frame memories that are used for an image process such as a scaling process. In such an image display device, the input image signal for the three-dimensional image is separated into an image signal for the left-eye image and an image signal for the right-eye image, and they are temporarily stored in the frame memory. The image signal for the left-eye image and the image signal for the right-eye image that are stored in the frame memory are alternately read and displayed at a regular timing, so that the frame sequential technique is actualized.
In the image display device including the frame memory, writing the image signal is performed at a timing (input synchronizing timing) synchronized with a synchronizing signal (input vertical synchronizing signal) that is input. On the other hand, reading the image signal is performed at a timing (output synchronizing timing) synchronized with a synchronizing signal (output vertical synchronizing signal) that is generated from a fixed clock in the image display device. Accordingly, the input synchronizing timing and the output synchronizing timing are asynchronous.
In the case where the input synchronizing timing and the output synchronizing timing are asynchronous, a phenomenon (passing phenomenon) sometimes occurs in which the read address passes the write address. If the passing phenomenon occurs, previous frames relative to display target frames are displayed after the passing phenomenon has occurred. Further, if the passing phenomenon occurs, the read order between the image signal for the left-eye image and the image signal for the right-eye image is sometimes reversed, because the left-eye image and the right-eye image are alternately displayed in the frame sequential technique.
Hence, in the frame sequential technique, the output vertical synchronizing signal is generated based on the input vertical synchronizing signal, and the timings of the writing and the reading are adjusted. Thereby, a control by which the passing phenomenon does not occur is performed. Such a control is called a synchronous mode control.
In the synchronous mode control, a generation counter to generate the output vertical synchronizing signal is reset by the input vertical synchronizing signal. When the generation counter is reset, a pulse signal is produced, and the output vertical synchronous signal is generated. Thereby, the start timings of the writing and the reading are adjusted, thus ensuring that the passing phenomenon does not occur. Incidentally, a control in which the output vertical synchronizing signal is not reset by the input vertical synchronizing signal and reading is performed using the output vertical synchronizing signal generated from the fixed clock in the image display device is called as an asynchronous mode control.